| Electronics > Projects, Designs, and Technical Stuff |
| Lab Power Supply - The Lost Current |
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| xavier60:
Also, does the discrepancy in current readings exist in CV mode? The loop should be stable in CV mode. |
| radoczi94:
--- Quote from: xavier60 on February 03, 2018, 02:52:04 am ---Don't modify it until you have found a definite reason for the problems. Have you checked for oscillations? When you checked the regulator PCB for a possible short bypassing the shunt, did you have all of the wires connected? --- Quote from: radoczi94 on February 02, 2018, 09:51:56 pm --- --- Quote from: TimNJ on February 02, 2018, 05:16:56 am ---Try a "diode-OR" to switch between the CV and CC control loops. This is a common and effective way to implement constant current limiting on a "normally CV" power supply. Check out the attached schematic of an HP6214A lab power supply. Also check out this Keysight article: http://powersupply.blogs.keysight.com/2012/07/how-does-power-supply-regulate-its.html --- End quote --- Thanks, will try it. Edit: One thing came into my mind. I am not shure, that I can put a diode in series with the CV opamp. What I could do is to move the diode on the CV opamp output (D7) and tie it's anode to the driver transistors' (Q4) base. --- Quote from: xavier60 on February 02, 2018, 12:01:13 pm ---There is a 5W 0.68 ohm resistor on the regulator PCB. I can't see the marking of the resistor next to it? Are these the shunt resistors? --- End quote --- Yes, it was in there, when I took the picture. When I put those 0,68R resistors,then I realised, that the "missing" 2 amps is more or less linear with the output load current. --- End quote --- --- End quote --- The wires were connected, just removed the shunts, when I was checking for shorts. I'm not sure, how to check for oscillations without a scope. My idea is to give this thing a try without th CC opamp installed. --- Quote from: xavier60 on February 03, 2018, 06:25:49 am ---Also, does the discrepancy in current readings exist in CV mode? The loop should be stable in CV mode. --- End quote --- Yes, it is stable in CV mode. The CC mode output current is pretty stable too. There are some smaller deviations, like 200mA or so, typically downwards. But it is stable, the meter does not moving at all, the current value just deviating a little bit every time the CC kicks in. |
| xavier60:
I was manly curious to know if there is a difference between output current and shunt current in CV mode also. |
| radoczi94:
That is one thing I did not measured, at least I don't remember. Will do that too, if I have time. You may probably noticed, my english is not the best, sorry for that.I'm trying. |
| C:
--- Quote from: radoczi94 on February 03, 2018, 10:08:33 am ---You may probably noticed, my english is not the best, sorry for that.I'm trying. --- End quote --- An doing good, Very simple, If you have problems to or from english just say so. :-+ oscillations without a scope With no scope the best you can do is trying to measure AC with meter. On DC mode with meter, only last digit value should change. Keep in mind that a Lab Power Supply is to help you when building new circuits & protect the circuit. Think of what happens when you are building a PWM dimmed LED circuit. At some point in this design, the power supply will see a load changing from almost no current to very high current very quickly. With volt meter, you can not see total effect on lab power supply. If you had a scope, a good test of power supply is a test load is one that changes a lot. Could help you a lot if you created a spreadsheet to record your measurements in. You should measure circuit many times with different loads. All these measurements will help you understand how the circuit is working and spot where it is not working as you think it should.. You are having problems with CC mode, Would be good to have all the measurements so you can compare before change to after a change. It might help you a lot if you make your schematic a lot better. You have Power paths to output. Power to run control circuit. Reference block. Output voltage sense. Output current sense. Voltage control circuit. Current control circuit. Output control circuit. Some quick cut & paste moves could make circuit easer to under stand. One way is to make top of schematic output & control with bottom power for control & reference.. Make a box around circuit that supplies power to control circuit. Box top is between D3 & X-1 Box right is between x2-2 & LED1 Move this box down below where X1-3 goes across schematic. Move Reference block. down also where X1-3 goes across schematic. Box left is between IC1 & X3-3 Box right is between X4-2 & R19 What I am calling Output Control is area above R26. Voltage Sense R21 & R27 could be vertical next to C17 Think these changes and a few more will change the schematic from cram it in to a more logical schematic. A more logical schematic can let you see problem areas. === |
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